XPS analysis of hydroxide ion surface reactions on reactions on CeF3 and LaF3 fluoride ion-selective electrodes

Lanthanum fluoride and cerium fluoride single crystals, used as the sensing membranes of the fluoride ion-selective electrode (F-ISE), were investigated for their hydroxide interference and surface reactions with OH− in high pH conditions. While these membranes show fast response and excellent Nernstian behavior over a wide concertration range in buffered F− solutions, they deviate from the theoretical slope at high pH. CeF3, in particular, exhibits a much larger deviation from the Nernstian slope and a substantially slower response to a F− activity change compared to LaF3. This larger deviation is due to more extensive and faster formation of hydroxo-complexes, with the release of a greater amount of the fluoride ion into the hydrated gel layer. The slower response of the CeF3 membrane after contact with OH− is due to the formation of CeIV oxide on the surface, which acts as a blocking layer to the exchange of F−.